1. [Technical Field]
The present invention relates to a control rod for a boiling water reactor.
2. [Background Art]
The control rod used in a boiling water reactor has a cruciform cross-section, is disposed in the cruciform gap formed among four square-pipe-like fuel channel boxes, and inserted into and removed from a core, thereby executing start-up and shutdown of the reactor and adjusting reactor power during the reactor operation. Conventional control rods include three types of structures.
In the control rod having a first structure, a plurality of hermetically-sealed hollow tubes are disposed in parallel with one another along the central axis of the control rod having a cruciform cross-section. Those tubes extend vertically, both ends thereof are sealed by end plugs, and neutron absorber is enclosed therewithin. All of the tubes are disposed in the four control rod blades, each composed of a thin metal sheath member having a U-shaped cross-section. An upper portion member is joined to an upper end portion of a tie rod and a lower support member is joined to a lower portion of the tie rod. An upper end of the sheath member is welded to the upper portion member, and a lower end of the sheath member is welded to the lower support member. Side ends of the sheath member having the U-shaped cross-section are welded to the tie rod. The hollow tubes are disposed inside the sheath member in parallel with one another. Typical description of such the first structure is made in detail in DESIGN, FABRICATION AND PERFORMANCE OF BORON-CARBIDE CONTROL ELEMENTS by H. A. Brammer et al. in the IAEA report (1964), “Physics and Material Problems of Reactor Control Rods”.
In a control rod having a second structure, solid-core metal plate members in which holes are horizontally made are used. FIG. 19 shows the second structural concept of the control rod 200. A length and a width of the metal plate member are the same as those of the control rod having the first structure. The metal plate member is provided with a plurality of horizontal holes extending from a side face of the control rod toward the center of the cruciform cross-section. In FIG. 19, horizontal holes 201 into which neutron absorber is filled exemplify those holes. Those horizontal holes 201 prescribe aligned columnar spaces for holding the neutron absorber. In the control rod having such a structure, the member provided with spaces for holding the neutron absorber also serves as a structural strength member of the control rod, making it possible to hold slightly more neutron absorber than the control rod of the first structure composed of tubes and sheath members. Typical descriptions of such the second structure have been made in detail in patent application publication No. 2002-533736.
A control rod having a third structure is provided with a plurality of tube members having a cross section of a deformed tube including four corner lobes which are disposed on a horizontal cross-section of constantly-thick side wall of a tube at intervals of 90° and useful for providing thicker sections for the tube. This control rod is provided with four blades formed by the plurality of tube members that are arranged in parallel with an axis of the control rod and welded one another, an end plug attached to each end portion of the tube members, a handle member joined to an upper end portion of the blade, and a lower portion support member joined to a lower end portion of the blade. FIG. 20 exemplifies such a structural concept.
In FIG. 20, the cylindrical tube 301 for holding neutron absorber filled in the deformed tube exemplifies an element of the aforementioned deformed tube. In the same manner as the control rod of the above second structure, in the control rod of this third structure, a member having spaces for holding the neutron absorber also serves as a structural strength member of the control rod, consequently, the member can hold slightly more neutron absorber than the control rod of the first structure composed of tubes and sheath members. The control rod of this third structure is disclosed in Japanese Patent Laid-Open No. 1(1989)-254895.
Furthermore, functions of common control rods used in boiling water reactor can be roughly classified into two types. The first type is a control rod used to shut down the reactor, which is designed to have relatively high neutron absorption reactivity worth. The second type is a control rod that is disposed at the control cell (a unit element composed of one control rod and four fuel assemblies which surround the control rod) of the reactor and is designed to adjust neutron flux distribution in the reactor thereby adjusting the reactor power.